How is Eustoma related to flowers sold as lisianthus?

There is a common misconception that the flowers sold commercially as "lisianthus" belong to the genus Lisianthius. In reality, the flowers sold under that name are members of the genus Eustoma.1

Eustoma is a genus of herbaceous plants within the Gentianaceae family, which is part of the Gentianales order. While these plants are widely marketed as "lisianthus," the true genus Lisianthius consists of shrubby, uncultivated tropical plants that are distinct from the popular cut flowers.21

Because Eustoma belongs to the Gentianaceae family, it is closely related to the gentians, which is why the plant is sometimes referred to as "prairie gentian".31